A controlled release tablet for the administration of medicinal agents over a prolonged period of up to about eight hours is described in U.S. Pat. No. 3,458,622, July 29, 1969, to John A. Hill. This patent discloses a compressed tablet for the prolonged release of a medicament containing that medicament in a core formed from a polymeric vinyl pyrrolidone, preferably polyvinyl pyrrolidone (PVP), and a carboxyvinyl hydrophilic polymer such as those marketed under the trademark Carbopol. The core material formed from the two polymeric substances provides the controlled release effect by forming a complex under the action of water or gastric fluid. This complex is gel-like in consistency and retards the diffusion of active ingredient from the tablet.
It has been found, however, that there is a tendency for an initial surge of medicament to occur so that the first amount of drug released may be larger than subsequently. This may be due to the short delay until water or gastric fluid acts on the polymeric blend and the gel, which provides the delaying action, forms.
It is therefore an object of this invention to improve the characteristics of controlled release tablets formed from a blend of polymers such as those in the Hill patent referred to above by reducing the tendency for an initial surge in the release of medicament.